This formula normalizes scores, that is it pulls a particular score (R) to the mean (C) if the number of reviews is not well above m. In other words, if a particular DZ has only a few reviews above the minimum required reviews to be listed in the top 10 (m), the average score is decreased a little if it is above the mean, or increased a little if it is below the mean in accordance with the normal distribution rule of statistics.

The formula normalizes the average rating of a relatively low rated DZ/Tunnel from 3.2 to 3.86 since the number of reviews (v=17) is only slightly above the minimum required votes (m=15) and the mean across all DZs (C=4.6) is quite high. If, in the future, this particular DZ/Tunnel gets more reviews, the difference between R and C will increase as the number of reviews increase. The idea is that the more reviews, the more representative the average rating is.